LTE-A/5G: Bring it on. Next-gen Soitec eSI90 wafers predict & improve RF performance.


The folks at SOI wafer maker Soitec have announced an amazing update to their RF wafer line-up, with what they’re calling their eSI90 substrate (read the press release here). As you might expect, it improves on their terrifically successful line of substrates for the RF chips in smartphones and other mobile devices. And now with this latest substrate, they’ve developed metrology that allows designers to predict the linearity of finished RF devices, ensuring they meet the demands for next-gen networks.

SOI wafers for RF are mainly 200mm (8”) in diameter. Soitec CEO Paul Boudre says they’ll continue to run at full capacity in 2015-2016. Additional wafers will also be available through Soitec’s partnership with Simgui in China.

(Courtesy: Soitec. (C) photo Christian Morel / www.morel-photos.com)
(Courtesy: Soitec. (C) photo Christian Morel / www.morel-photos.com)

How successful is this line? “Today, we estimate that more than one billion RF devices are produced each quarter using our eSI wafers,” says Dr. Bernard Aspar, senior vice president and general manager of Soitec’s Communication & Power Business Unit.  That’s for 2G, 3G and now 4G and LTE.

But with the advent of LTE-Advanced (aka LTE-A), 5G and Wi-Fi 802.11.ac (aka Gigabit Wi-Fi), RF designers need a whole lot more linearity in finished devices. That’s where eSI90 comes in.

Why? We’re looking at a 10x increase in smartphone data traffic (much of it due to high-def video) between 2013 and 2018, with average connection speeds jumping from 4Mbps to 7 Mbps.

SOIwafers_Soitec_lowres
SOI wafers. (Courtesy: Soitec)

But to achieve the throughput needed, designers are faced with draconian linearity requirements and far more complex front-end modules (FEM). The wafer substrate they start on has a major impact on the performance level of the final devices.

Seeing this coming, a few years ago Soitec teamed up with experts at the Université catholique de Louvain (UCL). Leveraging Soitec’s Smart Cut™technology, they developed and industrialized the addition of a “trap-rich” layer in high-resistivity (HR) SOI wafers (if you missed it, they wrote an excellent ASN piece explaining the technical details at the time – you can read it here).

The first generation of these trap-rich HR SOI wafers, which Soitec called eSI (for enhanced Signal Integrity), was a tremendous success from the get-go. Designers loved that the wafers enabled relaxed design rules, reduced process steps and gave them highly competitive performance and die cost, including a smaller area per function (well explained here).

So here’s what’s new about the new eSI90 wafers: they exhibit higher effective resistivity than first-generation eSI wafers, enabling a 10-decibel (dB) improvement in linearity performance in RF front-end modules to address the stringent new requirements of LTE-A smart phones.

Those eSI90 SOI wafers are designed to improve the RF performance of mobile communication components such as high-linearity switches and antenna tuners that are integrated in high-end smart phones for LTE-A networks using carrier aggregation. This enables multiple LTE carriers to be used together, providing higher data rates to enhance user experience.

To ensure that the new wafers would deliver on their promise, the Soitec team developed a new metrology metric, the Harmonic Quality Factor (HQF), to predict the expected RF linearity of finished ICs. We’ll have a more in-depth explanation of how this works coming up in ASN from the Soitec team. But for now, designers will appreciate that HQF correlates with the second harmonic distortion value of a coplanar waveguide deposited on the substrate. The new eSI90 wafers’ HQF maximum value is set to -90 decibel- milliwatts (dBm) compared to -80 dBm for first-generation eSI substrates. The lower limit on eSI90 wafers enables chipmakers to take advantage of design and process improvements to increase the RF performance of their chip designs and to meet MIMO (Multi-Input Multi-Output) and Carrier Aggregation LTE-A requirements, providing faster data connections.

The new eSI90 substrates are already under evaluation at leading chipmakers and foundries. Production-ready samples are now available from Soitec.

When it comes to next-gen mobile design, innovation really does start at the substrate level.

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